A database of sea lice counts from 33 sea water farms in Scotland, UK from the Firth of Cyde to Loch We, Sky and the Western Isles, was compiled. Data were assimilated for a 5-year period (1996-2000), involving counts on 70 824 fish (salmon). The pattern of abundance is reported for Lepeophtheirus salmonis. Lice numbers had strong seasonal and annual patterns. When stocking took place, after fallowing, sea lice numbers were very low and increased slowly to a level of around 7 lice per fish by the end of the first year of production. In the second year, a decline in counts was noted in the early spring but thereafter lice numbers increased throughout the year with fluctuations due to treatments with sea lice medicines. Mean lice abundance was not significantly influenced by coastal exposure or geographical region.